The concept of wood plastics composites (WPC) is starting to excite interest in Europe as methods of combining the two materials are developed and as the market continues to expand in the USA and other parts of the world. This new group of materials covers a wide range of polymer matrix types (including PP, PE and PVC) as well as a wide range of fillers and stiffeners (including wood flour, flax, jute and other cellulose based fibre fillers). The materials offer added strength, lower costs and a wide range of finishes but the high cellulose filler content of some products (up to 70%) may lead to some confusion in the market - is it wood or is it plastic? The plastics processing industry may change yet again as wood product companies transform themselves into composites companies.

The majority of the current applications are in the USA where significant advances are being made and finished products such as decking, cladding and window frames are already on the market. For the past 5 years the WPC market has grown at a rate of 100% per year and shows no signs of slowing down as new applications are developed for the materials. The majority of current applications are for the outdoor replacement of wood products but structural engineering applications are now being developed to use the improved physical properties of WPC.

The production of WPC typically uses a fine wood waste (sawdust in the 40 to 60 mesh range) mixed with various plastics. The powder is extruded to a dough-like consistency and the profile is then extruded through a single-step die (typically costing between ?00 and ?00) with no additional calibration and only a simple water bath for cooling.

Processing temperatures are less than 150 deg C, a temperature that allows high processing rates and low energy consumption. During production, the flow characteristics of cross-linked composite, through the extrusion system, permit the use of simple dies for even the most complex profiles. The simplicity of the die design means that lead times of six to eight weeks from design concept to production are usual.

Wood plastic composites, often used for such things as outdoor decking, are one of the fastest growing components of the wood composites industry. Some projections have suggested that these products, which were used for less than 1 percent of decking in the mid-1990s, may capture 20 percent of that market by 2010.

"Composite products made from wood and plastic are highly desirable for their low maintenance and ability to resist rot," said Kaichang Li, an associate professor in the OSU Department of Wood Science and Engineering. "But their use has been limited because of high cost and low strength, a result of inadequate adhesion between the wood fibers and plastic."

Fundamentally, Li said, this is because wood and plastic are like oil and water, and do not mix well. Wood is hydrophilic – it absorbs water – and plastic is hydrophobic, repelling it. A "compatibilizer," typically a polymer that bridges the interface between the wood and plastic in these products, improves stress transfer and increases their strength and stiffness.

The new wood plastic composites use superior compatibilizers developed in Li's laboratory, and an innovative technology for mixing wood and thermoplastics such as nylons, in which the melting temperature of the plastic is higher than the wood degradation temperature.

With this approach, the new wood-plastic composites can use very inexpensive plastics such as those found in old carpet fibers – about 4.4 billion pounds of which are now wasted every year, going into landfills where they are extremely slow to biodegrade and pose a significant waste disposal problem.

They could also open the door for improved utilization of low-grade woody biomass from needed thinning of Oregon forests, which is increasingly being done to improve forest health and prevent catastrophic wildfire. A better "value added" use for that wood fiber could be important, experts say.

The technology may prompt a major expansion of the wood plastic composite industry into new types of products and uses, experts say. In particular, such products may help further replace wood treated with chemical preservatives, some of which have already been banned due to health and environmental concerns.